All of them can do as much damage and self healing during scorpion as a guardian druid?

Nature's Vigil isn't guaranteed to heal the druid, self healing wise it's negligible at best. Other tanks have much better cds they can pop before entering scorpion survival wise. The %dmg boost is what makes them the best at it.

What's next best to do it after a gaurdian druid? Warrior and then Brewmaster(brewmaster has to not be terrible though and know how to build up 30+s of shuffle before popping it) then Pally and then DK is by far the worst. All tanks are capable of doing it if they know what they're doing though.

Ok, the question i asked was not "Which tanks are viable", it was which tank is the next best after a bear to do the scorpion.

Druid > Warrior (Avatar etc) > DK > Monk > Paladin (sadly the -15% dmg debuff monks have because REASONS applies to the scorpion form aswell). Just ask yourself, which class has the best/longest defensive cooldowns (to avoid getting killed), and then, which class also has the strongest DPS cooldowns in terms of increasing damage by a percentage.
Note that paladins just generally don't do a ton of dmg, and have no riposte to boost their output, like DKs/Warrs do.

Druid > Warrior (Avatar etc) > DK = Paladin > Monk (sadly the -15% dmg debuff monks have because REASONS applies to the scorpion form aswell). Just ask yourself, which class has the best/longest defensive cooldowns (to avoid getting killed), and then, which class also has the strongest DPS cooldowns in terms of increasing damage by a percentage.
Note that paladins and DK's are pretty close due to DK's riposte vs paladin's avenging wrath.

I actually forgot about this, yea my list was basically survival wise. Damage wise monks are the worst by far.